1. Field of the Invention
The present invention relates to a developing device which develops an electrostatic latent image formed on an image bearing member by using a toner.
2. Description of the Related Art
As a conventional developing device, as illustrated in FIGS. 5A and 5B, a developing container 1200 is separated by a partition wall 1209, and a developing chamber 1206 and an agitating chamber 1207 are disposed up and down in a vertical direction, so as to save a horizontal space of an image forming apparatus. The partitioned developing chamber 1206 and agitating chamber 1207 communicate with each other at both ends to allow a developer t to flow therebetween.
At an upstream side of the agitating chamber 1207 in a developer conveyance direction, a replenishing port 1208 is provided. A toner replenished from the replenishing port 1208 is conveyed by a second conveying screw 1205 inside the agitating chamber 1207, while being agitated with a carrier of the developer t within the agitating chamber 1207.
In a communication portion S1 disposed at a downstream side of the second conveying screw 1205 in a conveyance direction, the developer t is pumped from the agitating chamber 1207 to the developing chamber 1206. The developer t pumped to the developing chamber 1206 is conveyed to a communication portion S2 provided at a downstream side of a first conveying screw 1204 in a conveyance direction by the first conveying screw 1204 inside the developing chamber 1206, and falls to the agitating chamber 1207. By repeating the above operation, the developer t is circulated.
In this case, the replenish toner needs to be sufficiently mixed and agitated with the carrier. Otherwise, a charging amount of the toner becomes insufficient. As a result, image unevenness or density difference occurs in a thrust direction due to insufficient agitation, or a toner which is not controlled by an electric field covers a white background.
Therefore, in order to sufficiently mix and agitate the toner with the carrier, it is preferable that an agitating distance be taken as long as possible. Also, when the toner is replenished to the developing chamber side, since the developing chamber faces a developing sleeve, it is feared that the replenish toner may be directly scattered. Therefore, it is preferable that the toner be replenished to the agitating chamber side. As the optimal position to meet the two points, the replenishing port 1208 is disposed at the upstream side of the agitating chamber 1207 in the developer conveyance direction.
On the other hand, a developing device, in which a developing chamber 1206 is disposed in a vertically lower portion and an agitating chamber 1207 is disposed in a vertically upper portion, is proposed (US2008085138A1). This configuration can save a horizontal space of an image forming apparatus and can also stably supply a developer to a developing sleeve. Since the circulation of the developer is affected by gravity, a larger amount of the developer exists in the developing chamber disposed in the vertically lower portion as compared with the agitating chamber disposed in the vertically upper portion. Thus, it becomes difficult to delay the supply of the developer from the developing chamber toward the developing sleeve.
However, in the configuration of US2008085138A1, as illustrated in FIG. 6, when a toner replenishing port 1208 is disposed at the upstream side of the agitating chamber 1207 in the developer conveyance direction, it is feared that the replenish toner and the developer may be insufficiently mixed and agitated.
That is, in the configuration illustrated in FIG. 6, the developer t is subjected to high pressure in a communication portion S1 in which the developer t is pumped from the developing chamber 1206 to the agitating chamber 1207. In the communication portion S1 in which high pressure is applied to the developer t, the replenish toner and the developer are most efficiently mixed and agitated.
However, since the toner replenished from the toner replenishing port 1208 is supplied to the developing sleeve without passing through the communication portion S1, the agitation of the toner becomes insufficient, and therefore, it is feared that image unevenness or density unevenness may be caused by charging failure or agitation failure.
In order to sufficiently perform the agitation between the replenish toner and the developer, it may be considered to lengthen an agitating distance or increase an agitating ability by the improvement of screws. However, the former increases a size of an apparatus, and the latter improves an agitating ability but sacrifices a conveying ability.